1. Field of the Invention
The invention relates to measurements of temperature and of the level of electrolyte, based on molten cryolite, in cells for production of aluminum by electrolysis of alumina dissolved in said cryolite and to the application thereof for determining the thickness of the molten electrolysis bath in these same cells.
2. Discussion of the Background
The management of modern electrolysis cells for production of aluminum according to the Hall-Heroult process requires continuous surveillance of the temperature and the volume of the molten electrolysis bath. The greater part of the electrolysis bath is in the molten state and constitutes the electrolyte in which the carbonaceous anodes are immersed, the solidified remainder of the bath forms lateral slopes and the crust which covers the free surface of the electrolyte. This electrolyte is essentially constituted by Na.sub.3 AlF.sub.6 cryolite and can contain various additives such as CaF.sub.2, AlF.sub.3, LiF, MgF.sub.2, and so forth, which have the effect of altering the melting point and electrochemical properties as well as the ability of the bath to dissolve the alumina.
The volume of electrolyte covering the layer of liquid aluminum in contact with the cathode in the base of the cell, or cathodic substrate, has to be sufficient to allow dissolving and rapid separation of the alumina which is introduced in the upper part of the cell. At the same time, it must not exceed a certain level above which it would disturb the thermal equilibrium of the cell and cause corrosion of the steel rounds to which the anodes are attached, and as a consequence pollution with iron of the aluminum produced or metal.
It is therefore advisable to periodically monitor the level of the electrolyte, which represents its volume, that is to say the level of the air/electrolyte interface. This measurement is also useful when combined with measurement of the electrolyte/metal interface, for determining, by difference, the thickness of the electrolyte, that is to say the thickness of the molten electrolysis bath.
In the same way, the knowledge of and constancy in the temperature of the electrolyte are very important, on the one hand for properly regulating the operation of the cell under continuous operating conditions such as to correspond to a thermal equilibrium between the power supplied and the power dissipated, and on the other hand to optimize the electrolysis process, particularly the Faraday yield, taking into account that a simple increase in the temperature of the bath by ten degrees celsius can lower the Faraday yield by 1 to 2%, while conversely, a lowering of the temperature of the electrolyte by ten degrees celsius can, in the temperature zone under consideration (about 950.degree. C), reduce the already weak solubility of the alumina in the cryolite and promote "the anode effect", that is to say polarization of the anode, with a sudden increase in tension at the limits of the cell and the release of a large quantity of fluorided products produced by the breakdown of the electrolyte.
These measurements of the temperature and of the level of the bath are currently carried out manually by an operator, who periodically opens the door or cell lids and dips an insertion pyrometer into the electrolyte to measure the temperature, then a steel rod to measure the level and the thickness of the electrolyte. It is not possible to use a probe continuously immersed in the electrolyte because of its highly aggressive nature. This method clearly has a number of disadvantages, in particular from the point of view of:
releases of fluorided gases into the surrounding atmosphere during opening of the door or the cell lids, PA1 working conditions which expose the operator to these releases of gas, PA1 the low frequency (1 measurement per 24 to 48 hours) of these measurements which are difficult to undertake, which does not allow sufficiently regular and accurate monitoring of the temperature and the level of the electrolyte with respect to the new demands of management of high intensity cells. PA1 greater precision in the measurements of temperature to .+-.2.degree. C. (instead of .+-.5.degree. C. by the manual method) and of the level of the electrolyte to .+-.5mm (instead of .+-.10 mm by the manual method) together with increased accuracy in the management of electrolysis cells because of the greater frequency of measurements, preferably every 30 minutes to 48 hours instead of every 24 to 48 hours, allowing elimination of abnormal measurements occurring, particularly during the transient operating conditions of the cell. PA1 a gain in productivity, consecutive with the elimination of the task of manual measurement, together with a very substantial improvement in working conditions in the proximity of the cells with the ending of the opening of the door or the lids. PA1 a) piercing of the crust of solidified bath and immersion into the electrolyte, through the aperture thereby created, of the extremity of a temperature probe to a sufficient depth until a temperature of at least 850.degree. C., and preferably 920.degree. C. is obtained, then maintaining the immersion of the probe for a pre-determined length of time, which is less than the time taken to establish the thermal equilibrium of the probe with the electrolyte, PA1 b) after optional withdrawal of the probe, determination of the temperature of the electrolyte by extrapolation of the temperature values established by the probe above 850.degree. C. and preferably 920.degree. C., according to a pre-established calibration data preferably in the form of a computation program, PA1 c) after optional clearing of the aperture for the passage of the probe previously created, and optional removal of the solidified bath deposit from said probe, measurement of the level of electrolyte in the cell from a reference or datum point by recording the variation in potential between the cathodic substrate and the probe, the position of which is determined by a potentiometer, and the potential of which increases, preferably rapidly and significantly, when the lower extremity of the probe or tip comes into contact with the electrolyte, PA1 d) optional raising of the probe and PA1 e) optional calculation of the level of the electrolyte by the sensor after establishment of potential/position signals from the tip. PA1 1.degree.) the increase in temperature of the temperature probe between 850.degree. C. and 1050.degree. C., the normal operating range, obeys a law of development over time, the asymptotic curve of which can be calculated by extrapolation of the curve obtained over a short period of time. PA1 2.degree.) only the last N acquisitions by the probe indicating a temperature higher than or equal to 850.degree. C., and preferably higher than or equal to 920.degree. C. have to be taken into account to determine the equilibrium temperature or measurement of temperature of the electrolyte by extrapolation. PA1 3.degree.) the number N of these temperature acquisitions (N.gtoreq.10), carried out generally every 0.1 to 60 seconds, is limited and thus defined by the condition of withdrawal from the electrolyte of the probe at above 850.degree. C. and preferably at 920.degree. C., which is a speed of increase in temperature below a pre-determined threshold, preferably between 0.1 and 10.degree. C. per second. PA1 this firstly relates to the depth of immersion of the probe, which should be defined precisely. Indeed a significant error can take place due to thermal losses by conduction and by radiation along the probe, as the temperature of the measuring point (at the end of the probe) is always less than that of the electrolyte under continuous operating conditions. The depth of immersion should be at least 1 centimeter. PA1 it also concerns the regular cleaning of the external surface of the probe, ensured by the crust-breaker which surrounds said probe and the vertical translation movement of which causes the detachment of the deposit of solidified bath. It is preferred that the lower extremity of the periodically immersed probe is regularly relieved of its deposit of solidified bath on its external surface. Because it increases both the thickness and the length of the probe, it can on the one hand alter the conditions of electrolyte/probe thermal exchange, and therefore the measurement of the temperature, and on the other hand the threshold for detection by the tip when it enters the electrolyte, and as a result the measurement of the level of electrolyte.
Even the recent prior art only provides very incomplete solutions to these problems, while totally neglecting the aspect of measurement of temperature and advocating methods for measuring the level or the thickness of the electrolyte, the precision of which is still debatable, and moreover involving the use of individual control of the height of the anode over the cells. Thus EP 0195143 describes a method for measuring the level of the electrolyte in an electrolysis cell, according to which one of the anodes passed through by a given current is progressively raised, the reduction in current is measured according to the increase in the distance between the poles, that is to say the height raised, and the height at which the current has reduced to a pre-determined fraction of its initial value is noted. After calibration, the level of the electrolyte can be deduced. For this, the initial distance between the poles and a geometric correction term are added to the distance travelled by the anode.
In fact, this method supposes a very high degree of homogeneity of the electrolyte, whereas its resistivity varies locally and over time with its composition, and particularly with the content of alumina dissolved. Furthermore, this method necessitates significant movement of the anode which can disturb the working of the cell when this operation is repeated too often.
In the same way, EP 0288397 describes a method for monitoring the additions of solidified bath to an electrolysis cell, consisting of periodically determining the thickness of the electrolyte HB, which is compared to a reference variable HC and then adjusted accordingly. To obtain HB it is necessary, in an intermediate step, to measure the level of the bath with respect to a fixed point of reference, and this measurement is carried out by means of a probe connected to a level sensor and equipped with a tip electrically connected to the cathode of the electrolysis cell. When the tip comes into contact with the air/electrolyte interface, a large increase is recorded in the tip/cathode potential. Regardless of the fact that this method does not provide any operating data for this intermediate measurement of level (frequency, precision and accuracy) taking into account particularly the disturbing effect of the deposition of solidified bath on the probe, it in no way deals with the essential problem of the measurement of the temperature of the electrolyte.
To summarize, no method or device according to the prior art completely and satisfactorily resolves the problem of precise and accurate measurement of the temperature and of the level of the electrolyte in cells for the production of aluminum by electrolysis in order to eliminate the usual manual measurements.